Fixed content-disposition error that occurs with filename set and new chrome version 

This fixes an issue where new version of Chrome 16.0.912.63 would give this error when passing a filename to prawnto:

  Error 349 (net::ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_DISPOSITION): Multiple
  Content-Disposition headers received. This is disallowed to protect against HTTP
  response splitting attacks

This error was resolved by wrapping the filename in the content-disposition header in quotes.
WAS: "Content-Disposition"=>"attachment;filename=Name of your file.pdf"

NOW: "Content-Disposition"=>"attachment;filename=\"Name of your file.pdf\""